I'm willing to bet you have weak upper O2 sensors & there's nothing wrong with your knock sensor. But if you want, the knock sensor is under the lower intake manifold in the back that is located under the upper intake manifold also called the upper plenum. The lower intake manifold is wedged between the 2 heads and is an enormous pain in the azz to get to. You can check the O2 sensors with a scanning tool that will show you the upper sensors in bank 1 & 2 bouncing electrical current from about .5v to 5v several times per second. Anything less show weak sluggish sensors. The resulting lean mixture causes a knock. Change the sensors and the knock goes away and so does the engine light after you reset it. It took me 6 months to find this problem after replacing everything else including the 6mth old O2 sensors fresh from China. Now I have Bosch. PS The lower sensors went 2 weeks later & I was told any more problems it's the fuel regulator. 2001 Frontier 3.3l v6. No problems since, it purrs like a kitten. I'd suggest the Bosch, it's a special design for that 3.3l engine.